1. Field of the Invention:
This invention relates to the recovery of uranium from underground ore deposits or bodies by in situ leaching and the subsequent processing of the enriched leaching solution to recover relatively pure uranium compounds therefrom.
2. Description of the Prior Art:
Efforts have been made in the past to recover mineral values from underground ore deposits by introducing various leaching solutions in order to avoid the costs and problems of mining, such as are involved in tunneling, blasting and hauling of ore to the surface and then processing the ore by various means as by grinding, ball milling and flotation, followed by chemical solution or pyrometallurgy to recover the desired minerals therefrom. The application of leaching solutions of various types to underground ore deposits has been attempted with results that have varied widely, and only a few have been particularly successful, as for example, in the recovery of sulfur and salt. One of the problems leading to a lack of success for leaching out other minerals has been the fact that such other mineral whose recovery by in situ leaching is desired is that they often comprise only a small proportion of the total volume of the soluble minerals and insoluble gangue in the underground ore body. Consequently, the leach solutions must penetrate deeply into masses of gangue for a small recovery of the desired mineral values. In addition, the leaching solutions quite often have reacted with or been contaminated by numerous other minerals than the ones particularly desired as well as by clays and salts. This arises because the contaminants have also been only too well dissolved by the leaching solution. Leach solutions so contaminated have necessitated much subsequent refining processing in order to separate effectively the desired mineral from the undesired materials. A third factor is that excessive amounts of expensive leaching materials are necessarily employed because large proportions thereof are either dissipated, as by reaction of leach acids with limestone or calcite, or else substantial volumes of the expensive leaching solutions escape or are trapped and lost in the crevices of the ore deposit and never recovered.
These problems of in situ leaching are particularly critical in the process of recovery of uranium which is present in small percentages in most ore deposits that are reasonably amenable to leaching in situ.
U.S. Pat. No. 2,738,253 issued Mar. 13, 1956, discloses an initial application of an aqueous solution of sodium chlorate to a uranium bearing ore body followed by an acid leaching solution, which latter may or may not have additional sodium chlorate present therein, in order to recover the uranium values. The inventors in this patent indicate the fact that these ore bodies are often associated with ferrous iron along with tetravalent uranium. Tetravalent uranium is relatively insoluble in the leaching solution. By employing the sodium chlorate, the patent teaches that oxidation of the ferrous iron to ferric iron and the tetravalent uranium to hexavalent uranium is accomplished so that the acid leaching solution will readily dissolve the uranium and render it available.
Other acid leaching solutions are known, as in U.S. Pat. No. 3,309,141 issued Mar. 14, 1967, which discloses the combination of sulfuric acid and sodium chlorate in a leaching solution for extracting uranium from uranium bearing ore. U.S. Pat. No. 3,309,140 issued Mar. 14, 1967 teaches the use of a leaching solution comprising from 5 to 25 grams per liter of nitric acid and from 0.5 to 2 grams per liter of sodium chlorate. It is taught that the sodium chlorate is employed in order to oxidize the tetravalent uranium to the more soluble hexavalent uranium ion. Chlorates and nitric acid are both relatively expensive and have other drawbacks due to their highly corrosive effects on metal valves, piping, etc.
A number of patents have disclosed the employment of sodium carbonate solutions for extracting uranium from underground deposits by a leaching operation. U.S. Pat. No. 2,964,380, issued Dec. 13, 1960 discloses the general concept of a leachant comprising a 3% sodium carbonate solution in water which when applied to crushed uranium ore will leach the uranium therefrom.
U.S. Pat. No. 2,896,930, issued July 28, 1959 states generally that an aqueous solution containing "less than 50 grams per liter of dissolved carbonates" is suitable for underground leaching of uranium ore. An "alkali metal carbonate" is mentioned as suitable for such leaching utility. This patent states generally that "It is advantageous to incorporate an oxidizing agent such as hydrogen peroxide in the leach solution." No specific data or any specific proportions of suitable compositions are given in this patent, other than the above quoted upper limit for unspecified carbonates. At the bottom of column 3, of this patent, it is suggested that the recovery of the uranium whether from the leaching solution or from an inorganic solvent into which it has been incorporated by solvent extraction, may be effected using an ion exchange resin.
Another patent disclosing the use of carbonates is U.S. Pat. No. 2,818,240 issued Dec. 31, 1957. This patent discloses that carbonate solutions comprising 5 to 14% of sodium carbonate, 2% sodium bicarbonate and 5% of sodium chloride form aqueous solutions that would be of a pH of 9.9 to 9.6, but that the sodium chloride reduces the pH to 9.3. This patent also teaches that aqueous solutions of a pH of 9.6 or slightly in excess are effective in leaching out more of the various carbonaceous materials in the ore deposit. The patent also teaches that the sodium bicarbonate depresses the pH, and then it states, "which is undesirable" to secure maximum leading of carbonaceous material as is desired. U.S. Pat. No. 3,708,206, issued Jan. 2, 1973, teaches the pumping of an oxygen bearing gas such as air into a uranium ore body in order to oxidize the uranium to the hexavalent state, and after many hours or days of exposure to the oxidizing gas, a leach solution of sodium carbonate or ammonium carbonate is pumped into the oxidized ore body. The patent teaches as desirable leaching solutions, those containing from 23 to 26 grams per liter of ammonium carbonate.
U.S. Pat. No. 3,792,903 teaches the recovery of uranium from underground ore bodies by introducing leachants comprising sodium carbonate and an oxidant which latter may comprise air, oxygen or hydrogen peroxide. No specific solution compositions are given except that the patent states that the sodium carbonate leaching solution to the oxidizing solution may be proportioned from 1:1 to 1:10 by volume.
U.S. Pat. No. 3,130,960, issued Apr. 28, 1964 teaches the use, as a leaching solution, of carbon dioxide gas impregnated water applied to ore deposits of uranium and vanadium. It is noted that such leaching solutions should comprise at least 20% of the maximum possible carbonation in which 100% equals 30 volumes of carbon dioxide per volume of water. These solutions are obviously acidic. Thirty volumes of carbon dioxide in one volume of water provides approximately 59 grams per liter of carbon dioxide, while 20% carbonation introduces about 12 grams of carbon dioxide per liter. This last patent also teaches that the leach solution, after it has passed through the ore body and brought to the surface, is treated with lime to precipitate the uranium and vanadium values.
From the above, it will be apparent that the leaching solutions have generally been relatively concentrated and have comprised either acids or alkali metal carbonates. U.S. Pat. No. 2,818,240 is the only patent that employs a bicarbonate, namely sodium bicarbonate, in a leaching solution. None of the references teaches the use of ammonium bicarbonate and none suggests employing dilute ammonium bicarbonate solutions, alone, or with a peroxide, for leaching uranium values from ore deposits.
The following articles, comprising papers presented at Geneva, Switzerland from September 1 to September 13, 1958 as part of the "Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy," published in Volume 3, "Processing of Raw Materials," are of interest with respect to the present invention:
1. "The Role of Process Development in Western United States Uranium Procurement" by J. W. Barnes--pages 183 to 190;
2. "Some Variations of Uranium-Ore Treatment Procedures" by E. A. Brown et al--pages 195 to 200;
3. "Kinetics of the Dissolution of Uranium Dioxide in Carbonate-Bicarbonate Solutions" by W. E. Schortmann and M. A. DeSesa, pages 333 to 344; and
4. "Extraction of Uranium from Solutions of Sodium Carbonate by Means of Anionic Exchange with Dowex Resin" by M. Urgell et al, pages 444 to 464.
Also note the article by F. A. Foward in the October, 1953 issue of the Canadian Min. and Met. Bulletin, entitled "Studies in the Carbonate Leaching of Uranium Ores."
However, none of this last-mentioned art discloses the use of dilute ammonium bicarbonate and peroxide leaching solutions for recovering the uranium.